1. Field of the Invention
The present disclosure relates to an automatic transmission.
2. Discussion of the Background
There is a known automatic transmission capable of providing eight forward gears using a first planetary gear mechanism for input, two planetary gear mechanisms consisting of second and third planetary gear mechanisms for changing the speed, and six engagement mechanisms (see, for example, Japanese Unexamined Patent Application Publication No. 2005-273768).
In the automatic transmission described in Japanese Unexamined Patent Application Publication No. 2005-273768, the first planetary gear mechanism for input is a double-pinion planetary gear mechanism that includes a first sun gear, a first ring gear, and a first carrier that supports a pair of first pinions. The pair of first pinions mesh with each other, one of the first pinions meshes with the first sun gear, and the other meshes with the first ring gear. The first carrier supports the first pinions such that they are rotatable about their respective axes and about the axis of the first sun gear.
This double-pinion planetary gear mechanism is also called a plus planetary gear mechanism or positive planetary gear mechanism because when the carrier is fixed the sun gear and ring gear rotate in the same direction. When the ring gear is fixed the sun gear and carrier rotate in different directions.
In the first planetary gear mechanism, the first sun gear is a fixed element fixed to a transmission case, the first carrier is an input element coupled to the input shaft, and the first ring gear is an output element that reduces the rotational speed of the first carrier, which is the input element, and outputs the rotation.
The two planetary gear mechanisms for changing the speed are of Ravigneaux type made up of a second sun gear, a third sun gear, a second ring gear integral with a third ring gear, and a second carrier that supports a pair of second pinions. The pair of second pinions mesh with each other, one of the second pinions meshes with the second sun gear and second ring gear, and the other meshes with the third sun gear. The second carrier supports the second pinions such that they are rotatable about their respective axes, one of the second pinions is rotatable about the second sun gear, and the other is rotatable about the third sun gear.
The above-described Ravigneaux planetary gear mechanisms form four rotational elements. These four rotational elements are arranged at intervals corresponding to the gear ratios in a collinear diagram (a diagram that illustrates a ratio among relative rotational speeds of rotational elements using straight lines). When the four rotational elements in the collinear diagram are defined as a first rotational element, a second rotational element, a third rotational element, and a fourth rotational element from one side in sequence, the first rotational element is the second sun gear, the second rotational element is the second carrier integral with the third carrier, the third rotational element is the second ring gear, which is integral with the third ring gear, and the fourth rotational element is the third sun gear.
The six engagement mechanisms include a first wet multi-plate clutch that releasably couples the first ring gear being the output element of the first planetary gear mechanism and the fourth rotational element consisting of the third sun gear, a second wet multi-plate clutch that releasably couples the input shaft and the second rotational element consisting of the second carrier, a third wet multi-plate clutch that releasably couples the first ring gear being the output element and the first rotational element consisting of the second sun gear, a fourth wet multi-plate clutch that releasably couples the first carrier being the input element and the first rotational element consisting of the second sun gear, a first brake that releasably fixes the first rotational element consisting of the second sun gear to the transmission case, and a second brake that releasably fixes the second rotational element consisting of the second carrier to the transmission case.
With the above-described configuration, the first gear is established by the coupling state of the first wet multi-plate clutch and the fixing state of the second brake, the second gear is established by the coupling state of the first wet multi-plate clutch and the fixing state of the first brake, the third gear is established by the coupling state of each of the first wet multi-plate clutch and third wet multi-plate clutch, and the fourth gear is established by the coupling state of the first wet multi-plate clutch and fourth wet multi-plate clutch.
The fifth gear is established by the coupling state of each of the first wet multi-plate clutch and second wet multi-plate clutch, the sixth gear is established by the coupling state of each of the second wet multi-plate clutch and fourth wet multi-plate clutch, the seventh gear is established by the coupling state of each of the second wet multi-plate clutch and third wet multi-plate clutch, and the eighth gear is established by the coupling state of the second wet multi-plate clutch and the fixing state of the first brake.
In the above-described known automatic transmission, the number of engagement mechanisms being in the coupling state or fixing state at each gear is two. Thus, the above-described automatic transmission has the disadvantage of a large friction loss resulting from dragging of the remaining four opened engagement mechanisms and a decreased efficiency of the automatic transmission.
In addition, to achieve 10 or more forward gears, it is necessary to add at least two engagement mechanisms. In this case, the number of engagement mechanisms that are opened at each gear is six or more, and the friction loss further increases.